The man is horrified when Castiel asks him to torture Alastair. He doesn't want to do it, he literally begs not to. He was forced into being a torturer as the only way he could stop his own torture. He 'liked' it because it was a break from suffering, not because he wanted to do it or got off from it. He is adept at it because he had to be. It's also one of the tools of the trade (hunting). The fact he is good at it is devastating. He has to slip into the same mindset that made him survive Hell just to get through it. That isn't Dean, that's a trauma response and a coping mechanism. That's dissociating from his true self to be what he has to be so get the job done or spare himself. And the latter isn't a weakness but he still hates himself for it. He hates torture and he hates who he has to be do to it. What he did in Hell haunts him for the rest of his life.

And if he does ever 'enjoy' a kill, it's a righteous feeling - he has killed one more monster, prevented more death, avenged innocents, avenged the people he loves. At best, this gratification could be considered non-sexual sadism, but if that's the case, Sam and every other hunter is also a sadist. I see it more as righteous fury and satisfaction.

Both Dean and Sam torture people for the job. There's that episode where they torture a man possessed by a demon for information about Lilith. They both do it with the same cognitive dissonance of it just being part of the job.

Dean was also hunting with John from a young age and burdened with knowledge of monsters and what John had to do during a time when Sam was still protected from the truth. Violence and death were a part of him from a horrifically young age. He had to become accustomed to it because he had no choice. His still-developing brain has to build itself around some true horrors and he STILL grew up to be a good man who dedicated his life to saving people.

mary is the girl that leaves you to rot

heaven works hard arranging fates for a millennia to make sure the special guy is born.

but because of all the hard work, they also overdo it with the being wanted & feeling empty bits that come with being chuck's special guy and michael's vessel. chuck wants that guy so bad it's imprinted onto the guy's very dna. michael wants that guy so bad the guy feels forever empty as long as he isn't actively michael's vessel.

the emotionally incestous and borderline sexual john-barely-keeping-it-concealed johndean stems from that. it is the cause of dean never seeing family and family boundaries healthy ever again.

dean's empty and dean's used to being wanted and he wants something good in his life for once. nothing and no one is better than his baby brother.

sam can't escape the wanting dean part even if he thought it worked for a while during stanford.

sam and dean dance around that one for years.

mary doesn't see her firstborn baby boy in adult dean, but she sees the already weird, none-of-my-fault relationship between sam and dean. it was broken when i found it, she's not making it worse. and dean feels so empty he is ready to accept sexual love with mary if she can't give him familial. and don't forget the gaping wound left by never-happened johndean that dean now gets to close by sleeping with mary.